This section introduces aspects that may help facilitate a better understanding of the invention. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is prior art or what is not prior art.
Switches are a common design feature in electronic systems. Typically, a switch is a three-terminal device having control, input, and output terminals. Depending on the control voltage, a switch is either in the ON state or in the OFF state. Ideally, when in the ON state, the switch presents a vanishingly low resistance across its input and output terminals and, when in the OFF state, the switch does not allow any leakage current to flow through it. A typical switch may be implemented using a MOSFET transistor, where a control voltage is applied to the transistor's gate to control whether an input signal applied to one channel terminal (i.e., source or drain) of the transistor is presented as an output signal at the transistor's other channel terminal. Speed and linearity performance of a switch are improved by applying the highest possible control voltage and by applying a MOSFET gate-to-source voltage that is independent of the voltage of the input signal applied to the switch. On the other hand, if the gate-to-channel voltages applied to a transistor-based switch are too high, then the transistor devices are susceptible to degradation and even failure over their operating lifetimes, especially for modern semi-conductor technologies that involve very small thin-oxide gate thicknesses.
Some switches are implemented with bootstrap circuitry that allows the control voltage applied to a switch to be greater than the power supply voltage. To avoid over-voltage conditions, the bootstrap circuitry needs a mechanism to limit not only the gate-to-channel voltage applied to the transistor used to implement the switch, but also the voltages across any pair of terminals of any transistor device that constitute the entire bootstrap circuitry. Unfortunately, conventional bootstrap circuitry is relatively complex, consuming significant layout area and involving relatively long design development time. A. M. Abo, “Design for reliability of low-voltage, switched-capacitor circuits,” Ph.D. dissertation, Univ. California, Berkeley, Calif., 1999 (“the Abo reference”), incorporated herein by reference in its entirety, shows, in FIG. 5.5, one example of a conventional bootstrap switch circuit having bootstrap circuitry requiring eleven transistors, one inverter (presumably implemented using another two transistors), and three capacitors.